1. Field of the Invention
The present invention relates to a ferrite core material for a resin-filled type carrier and a resin-filled type carrier used for a two-component electrophotographic developer used in copying machines, printers and the like, and an electrophotographic developer using the carrier, particularly to a ferrite core material for a resin-filled type carrier and a resin-filled type carrier which have a low true density, an elongated life, an easily controllable charging amount, etc. and a high strength, and generate little cracking, deformation and melting by heat and impact, and an electrophotographic developer using the carrier.
2. Description of the Related Art
The electrophotographic developing method is one which develops images by adhering toner particles in a developer to electrostatic latent images formed on a photoreceptor, and the developers used in this method are classified into a two-component developer composed of toner particles and carrier particles, and a one-component developer using toner particles alone.
As the developing method using a two-component developer composed of toner particles and carrier particles among such developers, although the cascade method, etc., was formerly used, the magnetic brush method using a magnet roll prevails at present.
In a two-component developer, the carrier particles are a carrier material which is mixed and agitated with the toner particles in a development box filled with the developer to impart a desired charge to the toner particles, and carries the charged toner particles to a photoreceptor surface to form toner images. The carrier particles remaining on a development roll holding a magnet are again returned from the development roll to the development box, mixed and agitated with new toner particles, and repeatedly used in a certain period.
The two-component developer, unlike the one-component developer, is one in which carrier particles are mixed and agitated with toner particles, charge the toner particles, and further have a function of transporting them, thus having good controllability in developer design. Therefore, the two-component developer is suitable for full-color developing devices requiring high-quality images, high-speed printing machines requiring reliability and durability of image sustainability, and the like.
The two-component developer used in such ways requires image characteristics, such as image density, fogging, white spots, gradation and resolution, exhibiting predetermined values from the early stage, and stably maintaining these characteristics without variation in them during a continuous printing period. For stably maintaining these characteristics, the stability of the carrier particles contained in the two-component developer is required.
As carrier particles forming a two-component developer, iron powder carriers such as iron powder coated on its surface with an oxide film and iron powder coated on its surface with a resin have conventionally been used. Such iron powder carriers have an advantage of easily providing images good in reproducibility in solid parts because of their high magnetization and conductivity.
However, since such iron powder carriers have high own weights and too high magnetizations, fusion of the toner to the surface of the iron powder carrier, so-called toner spent, becomes liable to occur by mixing and agitation with the toner particles in a development box. The occurrence of such toner spent reduces the available carrier surface area, becoming liable to decrease the friction chargeability with the toner particles.
The resin-coated iron powder carrier sometimes generates the charge leak due to exfoliation of the surface resin by stress during endurance, and exposure of the core material (iron powder) having a high conductivity and a low dielectric breakdown voltage. Such charge leak breaks electrostatic latent images formed on a photoreceptor, and generates brush strokes and the like on solid parts, causing inferior durability such as hardly providing uniform images. By these reasons, the iron powder carriers such as the oxide-filmed iron powder and the resin-coated iron powder come at present not to be used.
As described in Japanese Patent Laid-Open No. 59-48774 (Patent Document 1), resin-coated ferrite carriers have been recently often used, in which a ferrite core material, having a low true specific gravity of about 5.0 and a low magnetization, is used in place of iron powder carriers, and is coated on its surface with a resin, and the developer life has been remarkably elongated.
However, with evolution from an era of simple function copying machines to that of combined machines with recently progressing office network, and with the service systems shifting from a system where contracted servicemen periodically exchange developers, etc., for maintenance to a maintenance-free era, the market comes to have the much larger need for the elongated life of developers.
Further, offices come to accept full-color images and have the larger need for high-quality images, and the toner particle size becomes smaller to obtain a high resolution.
Responding to this, there is the need for quick charging of the toner in a desired charge, and the carrier particle size has shifted to the small particle size with a high specific surface area. When the particle size distribution is totally made to be of a small particle size, especially particles on the fine powder side become liable to bring about the phenomenon of the carrier particles scattering or adhering to a photoreceptor, which is a disadvantage of the two-component developer, and induce fatal image faults such as white spots. Therefore, the small size carrier must be controlled to have a narrower particle size distribution width.
For solving the problems as above, many magnetic powder-dispersed carriers in which fine magnetic powders are dispersed in resins are proposed in Japanese Patent Laid-Open No. 5-40367 (Patent Document 2), etc., to make the carrier particles light, and elongate the developer life.
Since the true densities of such magnetic powder-dispersed carriers can be decreased by reducing the magnetic powder amount, thus reducing the stress by agitation, the chipping and exfoliation of the coated film is prevented, providing stable image characteristics over a long period.
However, since the magnetic powder-dispersed carriers are coated on the magnetic powders with binder resins, the carrier resistance is high. Therefore, they have a problem of hardly providing a sufficient image density.
The magnetic powder-dispersed carriers sometimes raise problems that magnetic fine particles are solidified with binder resins, and drop off by the agitation stress and the impacts in developing machines, and that the carriers themselves break possibly due to their low hardness as compared with those of iron powder carriers and ferrite carriers conventionally used. Then, dropped-off magnetic fine particles and the broken carrier particles adhere to a photoreceptor, and sometimes cause image faults.
Additionally, the magnetic powder-dispersed carriers have a disadvantage of having a high residual magnetization and a high coercive force because of use of fine magnetic particles, and deteriorating the fluidity of the developers. Especially when magnet brushes are formed on a magnet roll, high-quality images can hardly be obtained, because the fluidity is bad due to the high residual magnetization and coercive force, and bristles of the magnetic brushes are hardened. Further, even when the carrier leaves the magnet roll, since the carrier magnetic aggregation does not come loose, and the carrier cannot be rapidly mixed with a supplied toner, the rising of the charge quantity is bad, raising a problem of causing image faults such as toner scattering and fogging in images.
Further, although the magnetic powder-dispersed carriers can be fabricated by two methods of the crushing one and the polymerization one, both have a problem of a high producing cost because the crushing method is bad in yield, and the polymerization method is complicated in the producing process.
A resin-filled type carrier in which voids in a porous carrier core material is filled with a resin is proposed as one in place of the magnetic powder-dispersed carriers. For example, Japanese Patent Laid-Open No. 11-295933 (Patent Document 3) and Japanese Patent Laid-Open No. 11-295935 (Patent Document 4) describe cores or hard magnetic cores, polymers contained in the pores of cores, and carriers including coatings which coat the cores. These resin-filled type carriers are supposed to provide carriers which have little impact, desired fluidities, wide ranges of friction charging values, desired conductivities, and certain ranges of the volume average particle sizes.
Here, Patent Document 3 supposes that as the core material, various appropriate porous solid core carrier materials such as existing porous cores can be used. It describes that especially important are the porousness and the desired fluidity, and lists the soft magnetization, the porosity indicated by the BET area and the volume average particle size as noteworthy properties.
However, the porosity of about 1,600 cm2/g in BET area as described in the example of the Patent Document cannot provide a sufficiently low specific gravity even when voids are filled with a resin, and cannot respond to the recently increasing need for the elongated life of developers.
The measurement principle of BET area is to measure the physical adsorption and the chemical adsorption of a specific gas, and has no correlation with a core material porosity. That is, even a core material scarcely having pores generally varies in the BET area depending on its particle size, particle size distribution, surface material, etc. Then, even if the porosity is controlled through the BET area measured in such a way, the core material cannot be said to be one which can be filled fully with a resin. When a large amount of a resin is tried to be filled in a core material with no porosity or insufficient porosity, but with a high BET area, the unfilled residual resin is present independently without adhering to the core material, floats in the carrier, generates much aggregation of the particles, deteriorates the fluidity, largely varies in charging characteristics on loosening of the aggregate in an actual use period, and soon, thereby making obtaining of stable characteristics difficult.
Additionally, the precise control of the specific gravity and mechanical strength of the carrier after resin filling is, needless to say, difficult only by simply controlling the porosity expressed by the BET area.
Further, a sponge iron powder used in example cannot achieve a sufficiently light weight even after resin filling, and cannot possibly achieve a desired elongated life.
Additionally, the Patent Document uses a porous core, and the total content of a resin filled therein and a resin coated on the surface thereof is set to be preferably about 0.5 to about 10 wt. % of the carrier. Further, in example in the Patent Document, the resins are at most 5 wt. % with respect to the carrier. Such a small amount of the resins cannot achieve a desired low specific gravity, is not at all different from conventionally used resin-coated carriers, and can only provide similar performances.
Further, the Cu—Zn ferrite used in example contains much amount of heavy metals, and must be anyway discarded even if the developer life is elongated, thus not in line with the current of the recent years' environmental load reduction.
Use of a hard magnetization core as described in Patent Document 4 brings about a disadvantage of worsening the fluidity of the developer because of its high residual magnetization and coercive force. Especially when magnet brushes are formed on a magnet roll, high-quality images can hardly be obtained because the fluidity is bad due to the high residual magnetization and coercive force, and bristles of the magnetic brushes are hardened. Further, even when the carrier leaves the magnet roll, since the carrier magnetic aggregation does not come loose, and the carrier cannot be rapidly mixed with a supplied toner, the rising of the charge quantity is bad, raising a problem of causing image faults such as toner scattering and fogging in images.
Japanese Patent Laid-Open No. 54-78137 (Patent Document 5) describes a carrier for an electrostatic image developer in which pores and recesses of the surface of magnetic particles which have a lower bulk specific gravity than substantially non-porous ones and which are porous or have a large surface roughness are filled with a fine powder of an electric insulation resin, and contends that a developer is provided by using the carrier, which developer has advantages that, with the carrier having little toner accumulation on the carrier surface, the developer has little change in powder characteristics and friction charging characteristics under varying temperature and humidity conditions, and has a constant image density with time lapse and no decrease in the image density.
However, when a fine powder is filled in pores of magnetic particles which are porous or have a large surface roughness, if the iron powder is used as described in example of the Patent Document, the fine powder is relatively easily filled, but is difficult to be filled in very fine voids as is the case with voids in ferrite core materials.
When a fine powder dispersed in a solvent is tried to be filled, if the core material is an iron powder, it can be filled relatively uniformly as described above, but if the core material is a ferrite one, the solvent only permeates voids of the core material, resulting in the dispersed fine powder present on the core material surface. The powder easily drops off by being subject to the mechanical stress in developing machines, leading to disadvantages of remarkable changes in the charging characteristics and resistance characteristics.
For example, an iron oxide powder (trade name: TEFV, manufactured by Powdertech Co., Ltd.) as described in example of the Patent Document has a rough and uneven surface, but does not have porousness as is the case with the present invention as described later, and is besides made of iron powder, so a sufficiently low specific gravity cannot be achieved.
As described above, even the carriers described in Patent Documents 3 to 5 cannot sufficiently satisfy the needs that the image density is sufficiently secured; the carrier adhesion does not occur over a long period; and high-quality images can be maintained.